Organic EL devices have the following characteristics: (i) the devices are spontaneously emitting devices, (ii) the devices can be driven by low direct current voltage, and (iii) various emission colors, such as red, green, blue and white, can be realized by selecting an organic EL material to be used or the structure of the devices. In recent years, therefore, attention has been paid to the devices as not only a next generation display technique but also a large-area illumination technique.
The structure of an organic EL devices is roughly classified into bottom emission types and top emission types. The bottom emission type has a structure wherein a transparent electrode made of indium tin oxide (ITO) or the like is formed on a support substrate having light transparency, made of glass or the like, and further an organic emitting layer and a counter reflective electrode are stacked thereon successively. Light generated in the organic emitting layer passes through the transparent support substrate to exit the device. Meanwhile, the top emission type has a structure wherein a reflective electrode is formed on a support substrate, and further an organic emitting layer and a counter transparent electrode are successively stacked thereon. Light generated in the organic emitting layer exits not from the side of the support substrate but from the side of the counter transparent electrode. For the top emission type, the following technique has been investigated: a technique of rendering its counter transparent electrode a light semi-transparent and semi-reflective electrode to make a micro-cavity structure, and further selecting the distance between the counter electrode and the reflective electrode to amplify the intensity of light generated from its organic emitting layer, thereby emitting a light having a high intensity. See Patent Document WO 01/39554.
White emission is indispensable for illumination. As a white emission organic EL device, there has been investigated a technique of forming the above-mentioned organic emitting layer by stacking plural emitting layers which exhibit different emission colors. For example, Japanese Laid-Open Patent Application 2002-272857 discloses a technique of providing an organic layer of two blue emitting layers and an orange emitting layer, as shown as “hole-transporting layer/blue emitting layer/orange emitting layer/electron-transporting layer”. Japanese Laid-Open Patent Application 2004-006165 discloses a technique of stacking emitting layers in the three primary colors of red-green-blue (RGB), as shown as “hole-injecting layer/hole-transporting layer/red emitting layer/blue emitting layer/green emitting layer/electron-injecting layer”.
In attempting to use an organic EL device as described above to fabricate a flat illuminating light source, the simplest structure is a structure wherein an organic emitting layer is sandwiched between upper and under, or lower, electrodes in such a manner that the whole of the organic emitting layer is covered. However, this structure has the following problems: (i) a drop in voltage is caused in the electrode sections, in particular, the transparent electrode section, whereby the current density in the whole emission section does not become uniform so that the luminance becomes nonuniform in the emission surface of the device; (ii) electric current flowing in the emission section concentrates on an interconnection section where the emission section is connected to a driving power source, so that Joule heat is generated; and (iii) when an electrical short is generated between the upper and under electrodes at some spot in the emission surface, applied current concentrates on this conductive spot so that the periphery of the conductive spot no longer emits light.
Problems (i) and (iii) can be solved by making matrix electrodes wherein electrode lines cross at right angles, and arranging auxiliary lines having low resistance along the matrix electrodes. However, problem (ii) is not solved.
When the current density generated when voltage V is applied to an organic EL device is represented by J, J and V have, for example, a nonlinear relationship of J=A·Vn wherein A is a proportional constant, and n>1. When the voltage V is raised, the current density J abruptly becomes large. When the area of the emission section is represented by S, the current flowing in the entire emission section becomes SJ. This current concentrates on the interconnection section connecting the emission section with the driving power source. Consequently, Joule heat is generated and the interconnection section deteriorates thermally.
One means for decreasing the Joule heat generated in the interconnection section is to lower the resistance of the interconnection section. However, the interconnection section inside an illuminating light source is spatially restricted. Thus, it is difficult to lower the resistance. For this reason, it has been desired to lower the current density J which flows in the organic EL emission section to apply an organic EL device to large-area illumination.
Techniques for lowering the current density J, have been disclosed. Japanese Laid-Open Patent Application 11-329748 discloses a technique of stacking organic emitting layers in the direction of electric conduction to provide an intermediate conductive layer therebetween. Stack type elements wherein organic emitting layers are stacked in the direction of electric conduction are disclosed in Japanese Laid-Open Patent Application 11-329748 and 2003-045676, among others.
The stack type elements, wherein organic emitting layers are stacked in the direction of electric conduction, each have a structure wherein a number of organic EL emitting layers are stacked to interpose connecting layers therebetween. The driving voltage therefor increases by N times, but the current value decreases by 1/N times. Consequently, Joule heat in an interconnection section can be lowered by 1/N. As the connecting layer for supplying carriers to the adjacent organic emitting layers, use of any of a thin film, an inorganic material, and an organic material which generates carriers are disclosed. However, in all methods it is difficult to realize carrier balance for emitting light evenly in N organic emitting layers. To obtain white emission important for illumination, it is necessary to stack organic emitting layers which exhibit different emission colors. In this case, it is further difficult to adjust carrier balance since the organic materials used in the emitting layers are different. The distance between the electrodes becomes large since the N layers are stacked, which causes the following problems: it is difficult to attain optimization of optical interference; the efficient high quantity light emission cannot be easily achieved; and the luminous efficiency cannot easily be increased.
Japanese Laid-Open Patent Application 11-329748 discloses a technique of arranging organic EL devices in series on an emission surface. A similar technique is disclosed in Japanese Laid-Open Patent Application 2004-342614. Wherein there are three lines whereby four organic EL devices are connected to each other in series, and those three lines exhibit emission colors different from each other. See FIG. 14. A structure wherein the three lines emit light rays of the three primary colors, blue, green and red, respectively, enables white light emission relatively easily, unlike the above-mentioned stack type. However, there remain problems that the uniformity of the white is insufficient, the efficiency for emitting white light is insufficient, and the driving voltage becomes too high.
An object of the invention is to provide an organic EL apparatus which at low total driving voltage emits excellent whiteness uniformity.